GPS World, February 2018
ANTENNAS b pattern and lower cross polarization and can therefore be expected to improve performance Dual feed antennas are common in applications where a balance between precision and cost is needed while four feeds are used in high end applications such as geodesy and reference stations The antennas under consideration here have been tuned to obtain optimal behavior at GPS L1 Galileo E1 band and have been simulated in an electromagnetic solver Ansys HFSS with an infinite ground plane assumption to resemble the large metallic body frame of aircraft structures The gain patterns of the different antennas at GPS L1 Galileo E1 central frequency f 1575 MHz are shown in FIGURE 1 As discussed earlier the pattern is not uniform 28 GPS WORLD WWW GPSWORLD COM FEBRUARY 2018 b over angle for the single feed solution On the other hand the four feed antenna shows improved pattern uniformity the pattern has fewer azimuth and elevation variations with the two feed solution providing intermediate results Phase patterns for the three antennas are shown in FIGURE 2 Here again the one feed solution exhibits more angular variation than the multi feed solutions It is interesting to notice how strong phase variations occur in the same regions where the gain pattern also varies strongly When considering the DGD the frequency dependence of the phase pattern will have to be taken into account according to Equation 1 To show the DGD variability with respect to the aspect angle the standard deviation of the DGD over a 20 MHz bandwidth has been calculated FIGURE 2 3D RHCP phase patterns at f 1575 MHz for a single feed antenna b dual feed antenna c four feed antenna a c a c FIGURE 3 3D standard deviation calculated over frequency of the DGD for a single feed antenna b dual feed antenna c four feed antenna
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